# Running ENVI-met

The user friendly interface of ENVI-met sometimes makes people forget that they work with a sophisticated climate simulation system. Normally, these kind of numerical models can only be run on complex and extremely expensive supercomputers. In more than 20 years of ENVI-met developement, we have invented various technologies to bring the scientific power of physical climate simulation models to your desktop. ENVI-met uses a complex system of optimized algorithms and intelligent troubleshooting allowing you to run dynamical 3D microclimate simulations on an arbitrary standard PC bought anywhere on the world running MS WINDOWS.

However, installing the software on your PC is the smallest step in using ENVI-met for microclimate analysis…

## Time after time...

As ENVI-met is a complex simulation system, it required a huge amount of computing power from your PC.

When you start to get in touch with ENVI-met, you should build small models, e.g. in the size of 30×30 grids. These models will run very quickly and you will get your results very soon. You will also get back negative feedback very soon, if your model cannot be simulated properly due to the selected settings or other issues.

As you run simple simulations, you will more and more understand how the system works and which are the triggers and settings you need to modify to control your simulation. You will also see, that a numerical climate model is a highly non-linear system. Small issues, uncertainties or changes can lead to large changes or problems in the overall simulation, which is the famous Butterfly Effect in Chaos Theory.

As a matter of fact, ENVI-met is not a “normal” computer program, in which every action has a clear result and the failure to produce a result or an error message is a software programming failure. In many cases, you will need a lot of thinking and analysing, why the model results are as they are. Besides a feeling for numerical models, this also requires knowledge in microclimate and atmospheric physics.

With growing expertise in ENVI-met, you will start to built larger model areas. Doing so, you will suffer the problem of $N^2$ complexity: A 30x30x20 model consists of 18.000 grid cells, but a 60x60x20 model will have 72.000 grid cells. While the number of cells “only” is four times higher, the complexity of the calculation rises by $N^2$. It is 324 Mil. relations for the 30x30x20 model, but 5.184 Mil. relations for the 60x60x20 model….

So, for the start, keep your models small and simple….

## Taking the long way...

Once you've got familiar with ENVI-met and made your first small models, you will start to built and run larger models. The execution time needed by ENVI-met can hardly be compared to any other software you might have used before. Like calculating and rendering a high-resolution animation sequenze, ENVI-met will need days to week to finalize a simulation. Even on up-to-date computers, running a day cycle (24 h) for a 250x250x30 grid cell model can easily take up to one week or more processing time.

During this time, ENVI-met will occupy one CPU core of your PC and about 3 GB of memory (32 Bit Version). Depending on your computer, you will be able to execute other tasks while ENVI-met runs in the background or run several instances of ENVI-met.

## Choosing the right computer...

When you run large models, you will occupy your computer, or at least parts of it, for a long time. Although -depending on your PC- it might be possible to run even large ENVI-met simulations on your PC and keep on using it for your daily work at the same time, this is not recommended. It will significantly slow down both the ENVI-met simulation and your daily work.

Best choice is to have an own computing PC reserved for your ENVI-met simulations. Once installed, it doesn't require its own keyboard or monitor and you can access it through Remote Desktop programs. The only thing you need to do is to copy your ENVI-met files onto the calculation PC and copy back the results once they are finished. (And, in additon, you should check from time to time if your simulation is still running…).

## Cloud Computing

Besides buying and owning your own PC, there is a second option: Cloud Computing.

Many people talk about “the cloud” in these days and “the cloud” has become a synonym for many different kinds of internet based computing and storing services. While many people use the potential benefits of storing data in the cloud via Dropbox, iCloud or whatever service, only few “normal” computer users realize so far that they can also compute in the cloud.

So, if you have a set of ENVI-met simulations to be run, but not enough or powerful computers available, cloud computing might be an interesting option compared to buying new computers, especially if your simulation demand is once-in-a-liftime. There are different cloud computing services out there, but by far not as many as e.g. cloud storage providers. The most prominent of them are probably Amazon's Elastic Compute Cloud (EC2) and Microsoft's Azure. You would be surprised how many of the 'independent' service providers you know from the web do not have their own server farms, but work with Amazon's or Microsoft's cloud services instead, including Apple's ICloud (which is Amazon's Elastic Cloud when it goes to servers).

### How does it work?

You do not need to be a computer scientist to use Cloud Computing. If you have ever used a Virtual PC engine such as Virtual Box or others, you more or less know everything you need to know about cloud computing.

Cloud computing is nothing else then setting up a new PC somewhere else based on a pre-build computer image, just like setting up a Virtual PC. The only difference is, that you will get a real server running the operating system of your choice and not an emulated one.

As ENVI-met runs on WINDOWS systems only, Microsoft Azure is the first choice, simply because they offer pre-configured machines running WINDOWS. Once registered, you can easily select between a number of recent WINDOWS versions (mostly the Server versions) plus you can select the hardware you would like to have. As “to have” means “to buy” this of course defines the cost of your cloud adventure. Selecting a PC with 12 cores and a i7 CPU for example will get pretty expensive. However, you will only be charged for the time the PC is actually up and working…

At the time, Microsoft offers a EUR 150 free depot for new registrants. That might be enough to run a couple of ENVI-met simulations for you. If you are part of the MSDN network, you will get a recharge of at least EUR 75 each month for a very small membership fee. For universities or similar there are even better options for computing at low cost, both for research and education.

Once you have chosen your PC configuration you can start to create 1 to 1.000 copies of the PC system you have chosen, this can even be done using the Powershell. You also need to configure a shared drive to which the model files and the calculated results will go.

Finally, you need to log in into your new computer(s) via Remote Desktop and install ENVI-met. (There might be ways to offer pre-configured ENVI-met machines but we have not figured that out so far.)

Then, you start your model and (hopefully) get your model results from some computer far away.

It is of course important -like on your local PC- to check the simulation progress. It would be annoying if your simulation has crashed and you would need to pay for a cloud computer doing nothing….

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